Chemical attachment of the hydrophilic polymer poly(ethylene glycol)(PEG), which is also known as poly(ethylene oxide) (PEO), to molecules and surfaces is of great utility in biotechnology. In its most common form PEG is a linear polymer terminated at each end with hydroxyl groups:HO—CH2CH2O—(CH2CH2O)n—CH2CH2—OHThis polymer can be represented in brief form as HO-PEG-OH where it is understood that the -PEG- symbol represents the following structural unit: —CH2CH2O—(CH2CH2O)n—CH2CH2—In typical form n ranges from approximately 10 to approximately 2000.
PEG is commonly used as methoxy-PEG-OH, or mPEG, in which one terminus is the relatively inert methoxy group, while the other terminus is a hydroxyl group that is subject to ready chemical modification.CH3O—(CH2CH2O)n—CH2CH2—OH mPEG
PEG is also commonly used in branched forms that can be prepared by addition of ethylene oxide to various polyols, such as glycerol, pentaerythritol and sorbitol. For example, the four-arm, branched PEG prepared from pentaerythritol is shown below:C(CH2—OH)4+n C2H4O→C[CH2O—(CH2CH2O)n—CH2CH2—OH]4
The branched polyethylene glycols can be represented in general form as R(-PEG-OH)n in which R represents the central “core” molecule, such as glycerol or pentaerythritol, and n represents the number of arms.
PEG is a well known polymer having the properties of solubility in water and in many organic solvents, lack of toxicity, and lack of immunogenicity. One use of PEG is to covalently attach the polymer to insoluble molecules to make the resulting PEG-molecule “conjugate” soluble. For example, Greenwald, Pendri and Bolikal in J. Org. Chem., 60, 331-336 (1995) have shown that the water-insoluble drug taxol, when coupled to PEG, becomes water soluble.
In related work, Davis et al. in U.S. Pat. No. 4,179,337 have shown that proteins coupled to PEG have enhanced blood circulation lifetime because of reduced rate of kidney clearance and reduced immunogenicity. Hydrophobic proteins have been described that gain increased water solubility upon coupling to PEG. These applications and many leading references are described in the book by Harris (J. M. Harris, Ed., “Biomedical and Biotechnical Applications of Polyethylene Glycol Chemistry,” Plenum, N.Y., 1992).
To couple PEG to a molecule such as a protein or on a surface, it is necessary to use an “activated derivative” of the PEG having a functional group at the terminus suitable for reacting with some group on the surface or on the protein (such as an amino group). Among the many useful activated derivatives of PEG is the succinimidyl “active ester” of carboxymethylated PEG as disclosed by K. Iwasaki and Y. Iwashita in U.S. Pat. No. 4,670,417. This chemistry can be illustrated with the active ester reacting with amino groups of a protein (the succinimidyl group is represented as NHS and the protein is represented as PRO-NH2):PEG-O—CH2—CO2-NHS+PRO-NH2→PEG-O—CH2—CO2—NH-PRO
Problems have arisen in the art. Some of the functional groups that have been used to activate PEG can result in toxic or otherwise undesirable residues when used for in vivo drug delivery. Some of the linkages that have been devised to attach functional groups to PEG can result in an undesirable immune response. Some of the functional groups do not have appropriate selectivity for reacting with particular groups on proteins and can tend to deactivate the proteins.
Attachment of a PEG derivative to a substance can have a somewhat unpredictable impact on the substance. Proteins, small drugs, and the like can have less biological activity when cojugated with a PEG derivative. For others, activity is increased.
Another example of a problem that has arisen in the art is exemplified by the succinimidyl succinate “active ester” mPEG-SS (the succinimidyl group is represented as NHS): 
The mPEG-SS active ester is a useful compound because it reacts rapidly with amino groups on proteins and other molecules to form an amide linkage (—CO—NH—). A problem with the mPEG-SS active ester, which was recognized by K. Iwasaki and Y. Iwashita in U.S. Pat. No. 4,670,417, is that this compound possesses an ester linkage in the backbone that remains after coupling to an amine such as a protein (represented as PRO-NH2):mPEG-SS+PRO-NH2→mPEG-O2C—CH2CH2—CONH-PRO
The remaining ester linkage is subject to rapid hydrolysis and detachment of PEG from the modified protein. Too rapid hydrolysis can preclude use of a PEG derivative for many applications. Several approaches have been adopted to solve the problem of hydrolytic instability. For example, mPEG succinimidyl carbonate has been proposed, which contains only ether linkages in the polymer backbone and reacts with proteins to form a conjugate that is not subject to hydrolysis.
It would be desirable to provide alternative PEG derivatives that are suitable for drug delivery systems, including delivery of proteins, enzymes, and small molecules, or for other biotechnical uses. It would also be desirable to provide alternative PEG derivatives that could enhance drug delivery systems or biotechnical products.